Flame ionization detector



March 12, 1968 L. E. GIUFFRIDA 3,

FLAME I ONI ZAT ION DETECTOR Filed Jan. 8, 1965 2 Sheets-Sheet l FROM 21 \I 'coLuMN l FIGJ PARATHION METHYL STEARATE INVENTOR FIG- 2 LAURAE.6IUFFRIDA ATTORNEY March 12, 1968 L. E. GIUFFRIDA FLAME IONI ZAT IONDETECTOR Filed Jan. 8, 1965 FIGS 2 Sheets-Sheet 2 RONNEL ETHION TRITHIONFIG.5

DIAZINON PARATHION FIG.7

INVENTOR LAURA E.GIUFFR|DA ATTORNEY United States Patent Ofifice FLAMEIONIZATION DETECTOR Laura E. Giutfrida, Arlington, Va., assignor to theUnited States of America as represented by the Secretary of theDepartment of Health, Education, and Welfare and the Secretary of theDepartment of Agriculture Filed Jan. 8, 1965, Ser. No. 424,443 4 Claims.(Cl. 23--254) ABSTRACT OF THE DISCLOSURE A hydrogen flame ionizationdetector, for use in gas chromatography, the heated electrode beingcoated with a fused alkali metal salt. Coating the electrode in thismanner renders the detector selectively specific tophosphorus-containing organic compounds in mixtures, so that higher,more characteristic peaks are indicated when these mixtures areanalyzed, while smaller peaks, or none at all, are indicated for thenon-phosphorus-containing compo nents of the mixtures.

A non-exclusive, irrevocable, royalty-free license in the inventionherein described, throughout the world for all purposes of the UnitedStates Government, with the power to grant sublicenses for suchpurposes, is hereby granted to the Government of the United States ofAmerica.

This invention relates to a flame ionization detector for use in gaschromatography. More particularly, it relates to a novel thermionicdetector having an exceptionally high selective sensitivity tophosphorus in an organic compound.

Gas chromatography has become an extremely valuable research tool forthe detection, identification, and measurement of minute amounts ofchemicals in mixtures and the like. Since about 1958, flame ionizationdetectors have been used in conjunction with gas chromatograph apparatusfor this purpose. These detectors operate on the principle that, when anorganic compound is burned in a hydrogen flame, the electricalconductivity of the latter increases. Thus, when a gas is passed throughthe chromatograph column it will elute substances which have beenadsorbed on it and, when mixed with hydrogen and burned, the resultingflame will have a higher electrical conductivity than if the carrier gasalone is burned with the hydrogen. Different substances not only producedifferent changes in conductivity but also are eluted from the column atdifferent rates. The time required to elute the particular material(referred to as the retention time), together with the relative increasein conductivity of the flame, can be used to determine the amount of andto identify the adsorbed material.

In general, the ionization detectors available prior to my invention,and currently in use, comprise a combustion chamber having an air-inletto provide the necessary oxygen for combustion, a gas inlet which isconnected to the outlet end of the chromatograph column, and an inletfor introducing hydrogen. The hydrogen inlet and the gas inlet areconnected by means of ducts to a common duct which terminates in a jetwithin the combustion chamber. The mixture of hydrogen and carrier gasis ignited and the flame beats a chemically inert wire loop, usuallymade of platinum. If a voltage is impressed across the jet and the wireloop, the ions produced in the hydrogen flame will increase theelectrical conductivity of the latter and thereby increase the currentflow. The greater the number of ions the greater will be theconductivity of the flame. By connecting the detector to anelectrometer, as is well known in the art, the differences in currentcan be detected and measured. Furthermore, by using a recording elec-3,372,994 Patented Mar. 12, 1968 trometer, the changes in current can betraced on a moving tape so as to plot the current intensities againsttime. As succeeding substances are eluted from the chromato graph columnby the carrier gas and the latter is burned in the flame detector, thetraced curve will show peaks of varying height and sharpness. Thisapparatus is so sensitive that it can detect amounts as small as one ortwo micrograms.

In recent years, the use of organo-phosphorus insecticides has becomequite extensive. However, while these substances are highly effectivepesticides, they are also extremely toxic to warm-blooded animals,attacking the nervous system. For this reason, it is necessary not onlyto take great precaution in applying these insecticides, but also toinsure that no residues remain on edible vegetables and fruits. Becauseof the great toxicity of even small amounts of these pesticides, gaschromatography has become a valuable technique in detecting them.However, while the presently available flame ionization detectors aregenerally suitable for this purpose, they are not sufficiently specificand will also record peaks for other substances which may be present inmixtures being analyzed. In some instances the peaks for thenon-phosphorus-conraining components are far more well-defined thanthose for the compounds being sought and, often, depending on the natureof the phosphorus compound, the latter does not even produce asignificant peak.

Accordingly, one object of the present invention is to provide a flameionization detector which is selectively specific tophosphorus-containing organic compounds. Another object is to providesuch a detector which is more sensitive to phosphorus-containing organiccompounds than conventional detectors and which will indicate higher,more characteristic peaks when these compounds are analyzed. Stillanother object is to provide a detector which gives smaller peaks, ornone at all, for the nonphosphorus-containing components of mixtures.Other objects will become apparent to those skilled in the art from thedescription of my invention which follows.

In general, the foregoing objects are achieved by substituting, for theheated electrode of the conventional hydrogen flame detector one thathas been coated with a fused alkali-metal salt. Although sodium sulfateproduces the best results, and is preferred for that reason, other saltscan also be used. Among the latter are lithium sulfate, sodium chloride,sodium phosphate, potassium sulfate, potassium chloride, potassiumphosphate, potassium nitrate, rubidium sulfate, and cesium sulfate.

My modified flame ionization detector is used in the same manner asthose previously available, as will be readily apparent to those skilledin the art.

In order that the invention may be better understood, reference is madeto the following detailed description and to the accompanying drawingsin which:

FIGURE 1 represents a partially schematic cross-sectional view of mynovel detector;

FIGURE 2 shows the curve produced when a mixture containing lindane,parathion, and methyl stearate is analyzed by the conventional hydrogenflame ionization detector;

FIGURE 3 shows the curve produced when the same mixture is analyzed bymy novel sodium thermionic detector;

FIGURE 4 shows the curve produced when .a first eluate of a crop extractcontaining a mixture of Diazinon, ronnel, parathion, ethion, andTrithion is analyzed by the conventional hydrogen flame ionizationdetector;

FIGURE 5 shows the curve produced when a duplicate first eluate of theabove crop extract is analyzed by the sodium thermionic detector of myinvention;

FIGURE 6 shows the curve produced when a second eluate of the sameextract is analyzed by means of a conventional detector; and

FIGURE 7 shows the curve produced when a duplicate second eluate isanalyzed by the sodium thermionic detector.

Referring to FIGURE 1 the device will be seen to resemble a conventionalhydrogen flame ionization detecto' having a base or support 1, providedwith air inlet 2, hydrogen inlet 3, and gas inlet 4. The latter isconnected in the usual manner to the outlet of a conventional gaschromatograph column (not shown). As can be seen, inlets 3 and 4 areboth connected to a tube 5 which terminates in a jet tip 6. One featureof my invention is the spiral electrode 7 which surrounds the flame 8and is concentric with jet 6. This electrode is connected to conductor 9by any suitable means, or it may be integral therewith, and may bemaintained in place by winding several turns of conductor 9 into a coil10 about tube 5. It will be apparent, however, that the precise mannerof mounting electrode 7 is not important and that any alternativemechanical means for locating it and maintaining it around the flamewould be satisfactory. The essential feature of the present inventionresides in providing electrode 7 with a fused coating of sodium sulfateor of any of the other alkali-metal salts mentioned above.

A further feature of my invention resides in the electrode 11. Thiscomprises a perforated, open-ended metal cylinder, preferably ofplatinum, which is rigidly mounted on a stiff wire conductor 12 and isconcentric with electrode 7. Electrode 11 may be welded to conductingsupport 12 to provide maximum strength and rigidity and maximumelectrical conductivity through the joint. Conductor 12 is secured tobase 1 in any suitable manner as by providing a very snug fit through ahole in the base.

The manner of supporting electrode 11 is not critical and any othermechanical means to accomplish this would be satisfactory. Conductors 9and 12 are both long enough to extend out of the device, as shown, sothat they can be connected to an electrometer (not shown) in the usualmanner. The electrodes and the jet are enclosed in housing 13 which ismounted on base 1 and is secured thereto by any appropriate means, suchas brackets 14. Combustion products and other gases are exhausted fromthe interior of the device through vented cover 15.

The results obtained by the use of my novel detector are illustrated inthe following examples:

Example 1 A mixture containing 1 microgram of lindane (a chlorinatedcyclohexane), 1 microgram of parathion (an organo-phosphorus compound),and 2 micrograms of methyl stearate were adsorbed on a gas chromatographcolumn, eluted with nitrogen in the usual manner, and then analyzed by acommercially available hydrogen flame detector and also by the sodiumthermionic detector of the present invention. FIGURE 2 shows theresponse of the hydrogen flame detector at an electrometer setting of 1Oamperes full scale (A. FS). As can be seen from the figure, each of thethree components of the mixture produced sharp, well-defined peaks. Thecurve in FIG- URE 3 shows the same sample analyzed by the novel sodiumthermionic detector at a setting of 3 I A. FS. It was necessary toattenuate 300 times to keep the respouse for parathion on scale. InFIGURE 3, lindane is barely detected and methyl stearate does not appearat all. By comparing the records from both detectors the presence of aphosphorus-containing compound can be readily demonstrated and theamount determined in the known manner.

Example 2 In this example, Diazinon and ronnel, 0.05 part per million(ppm) each, and 0.1 ppm. each of parathion, ethion, and Trithion wereadded to a 100 gram sample of broccoli. All are organicphosphorus-containing substances. The sample was then prepared for gaschromatography by the method described by Mills of al. in the Journal ofthe A.O.A.C., vol. 46, page 186 (1963). FIG- URE 4 shows the analysis ofan aliquot of the first eluate of the sample, representing 1 gram ofbroccoli, by the hydrogen flame detector at l() A. PS. As can be seen,there was practically no response. When a duplicate aliquot of the firsteluate was analyzed by the sodium thermionic detector at a sensitivityof 3X10 A. FS, sharp peaks due to ronnel, ethion, and Trithion appeared.These are shown in FIGURE 5.

Diazinon and parathion are recovered from the chromatograph column inthe second eluate. When the latter was analyzed by the hydrogen flamedetector at an electrometer setting of lO A. PS, the two major peaksshown in FIGURE 6 represented organic compounds that did not containphosphorus. However, when a duplicate analysis of the second eluate wasmade, using the sodium thermionic detector at a setting of 3X10 A. FS,Diazinon and parathion gave the sharp responses shown in FIGURE 7.Nitrogen was used as the eluting gas in this example.

As previously indicated, my invention resides in the novel detector.This was made by modifying a hydrogen flame detector of basicconventional design which was provided with an electrode assembly thatpermitted electrodes to be readily interchanged. Although the inventionhas been described using a coated spiral as the positive electrode, thelatter can also be in the form of a ring or circular gauze. In eachinstance, the electrode is preferably of platinum and carries a coatingof the sodium salt fused on the surface. A diameter of about 5 mm. wasfound satisfactory. While larger electrodes can be used, they wouldrequire larger flames. Similarly, smaller electrodes can be used, butthese would require more critical centering about the jet. The electrodeis positioned approximately 2 mm. above the jet. The shape of theelectrode does not appear to be critical, as responses from all threeare comparable. However, the spiral wire is preferred as being the mostconvenient to use.

The electrodes are prepared by placing a drop of saturated aqueoussolution of the alkali-metal salt on the wire and heating it gently in aflame to dry. This can be repeated a sufiicient number of times (e.g.,three times) until a layer of the desired thickness is formed. The wireis then heated to a temperature high enough (e.g., red heat) to causethe salt to fuse and distribute itself over the surface. When cool, theelectrode can be assembled in the detector.

The novel detector can be used with any commerciallyavailable gaschromatography equipment. Operating conditions, techniques, andcalibration of the apparatus are well known to those skilled in the artand need not be described in detail here.

Having described my invention, what I claim is as follows:

1. A flame ionization detector for use in gas chromatography comprising:

(a) a base member and a housing mounted thereon;

(b) gas-burning means mounted on the base within said housing;

(c) means connected to said gas-burning means for conducting a stream ofhydrogen thereto;

(d) means also connected to the gas-burning means for conducting astream of carrier gas and eluate from a chromatograph column to saidgas-burning means;

(e) means for introducing air into the interior of the housing;

(f) a first electrode, having a coating of a-fused alkalimetal saltselected from the group consisting of so dium sulfate, lithium sulfate,sodium chloride, sodium phosphate, potassium sulfate, potassiumchloride, potassium phosphate, potassium nitrate, rubidium sulfate, andcesium sulfate, mounted in said housing in proximity to said gas-burningmeans;

(g) a second electrode mounted in said housing in proximity to the firstelectrode; and (h) electrically conductive means connected to each ofsaid electrodes. 2. The detector of claim 1 wherein the first electrodehas a coating of fused sodium sulfate thereon.

3. A flame ionization detector for use in gas chromatography comprising:

(a) a base member and a housing mounted thereon;

(b) gas-burning means mounted on the base within 10 said housing;

(c) means for introducing a stream of hydrogen into said gas-burningmeans;

(d) means for also introducing a stream of carrier gas and eluate from achromatograph column to said 10 gas-burning means;

(e) means for introducing air into the interior of the housing;

(f) a first electrode, having a coating of a fused alkalimetal saltselected from the group consisting of sodium sulfate, lithium sulfate,sodium chloride, so-

dium phosphate, potassium sulfate, potassium chloride, potassiumphosphate, potassium nitrate, rubidium sulfate, and cesium sulfate,mounted in said housing and disposed concentrically about saidgasburning means;

(g) a second electrode mounted in said housing disposed concentricallyabout said first electrode; and (h) electrically conductive meansconnected to each of said electrodes. 4. The detector of claim 3 whereinthe first electrode has a coating of fused sodium sulfate thereon.

References Cited UNITED STATES PATENTS 3,129,062 4/1964 Ongkiehong eta1. 23-232 C MORRIS O. WOLK, Primary Examiner. JOSEPH SCROVRONEK,Examiner. R. M. REESE, Assistant Examiner.

